This invention relates generally to rotary drilling equipment and methods and, more particularly, to devices and methods for drilling a borehole at an angular deviation from vertical.
In the oil and gas industry, it is often necessary to drill a well at an angle from vertical. Due to the flexibility of long runs of drill pipe (drill string), deviations from vertical of very large magnitudes may be obtained. However, because of the same flexibility, specific angular deviations are difficult and costly to maintain. It is also difficult as well as expensive to vary the angular deviation at different points in the drilling operation.
In directional drilling operations, a well is initially drilled to a specific depth in order to establish a desired angular deviation from vertical. This initial step is accomplished by means well known in the art. Once a desired deviation is established, it may be continued to a target area, or it may be varied. Varied deviations occur when the driller must steer the drill string around a formation and then return to vertical or to some other desired direction (commonly called an s-type hole). Once a deviation is established, the side of the borehole nearest to a true vertical bore is termed the "low" side of the hole and is used as a reference point.
A common problem encountered in directional drilling is the variance from the desired deviation caused by the rotational forces exerted on the drill bit. This phenomenon is known as "walk". The drill bit will generally tend to walk to the right of the low side of the hole, although other directions of walk are possible depending on the circumstances. When a directional well is initialized, the hole is often started to the left to compensate for right hand walk during the process of establishing the desired deviation. Various devices have been designed to help eliminate undesired deviation while guiding the bit in a desired direction:
______________________________________ U.S. Date Patentee Pat. No. Issued Title ______________________________________ Baker, et al 4,416,339 11/22/83 Bit Guidance Device Method Jeter 4,319,649 3/16/82 Stabilizer Farris, et al 4,305,474 12/15/81 Thrust Acterated Drill Guidance Device Hamilton 4,220,213 9/02/80 Method and Apparatus for Self Orienting a Drill String While Drilling a Well Bore Nixon, Jr. 4,185,704 1/29/80 Directional Drilling Apparatus McMahon 3,825,081 7/23/74 Apparatus for Slant Hole Directional Drilling Garrison, 3,561,549 2/09/71 Slant Drilling Tools et al for Oil Wells Farris, et al 3,092,188 6/04/63 Directional Drilling Tool Page, et al 2,891,769 6/23/59 Directional Drilling Tool ______________________________________
Simplicity in design and application is a key factor in the success of such designs. Several of the above listed designs require pistons actuated by hydraulic pressure or other complicated mechanical actuation. Other designs can only be used with a down hole motor rather than conventional rotary drilling equipment. Such designs are therefore expensive, prone to mechanical failure and restricted in their applications.
A device which is simple in mechanical operation, self-actuating, self-orienting and which attempts to correct undesired deviation is illustrated by the Hamilton patent listed above. The Hamilton device comprises a longitudinally eccentric tubular member which attaches rotatably around a mandrel within the drill string. A projection is provided, extending radially from the tubular member to a greater extent than the diameter of the drill bit. When the device is placed between the drill string and the drill bit, the heavier portion of the tubular member theoretically rotates to the low side of the borehole, positioning the projection such that it urges the drill bit in an opposite direction from that to which the bit has a tendency to walk.
One problem which occurs with the Hamilton device results from the fact that all boreholes do not retain a diameter greater than that of the drill bit. Many formations tend to collapse toward the center of the borehole, causing the diameter of the hole to shrink. When this occurs, the Hamilton device will become stuck in the hole, losing its self-orienting feature and necessitating expensive recovery operations. This sticking is due in part to the fact that the projection in Hamilton extends past the diameter of the drill bit. The general sectional configuration of the Hamilton device does not promote smooth rotation within the confines of the borehole. Also, the Hamilton device cannot be used to intentionally change the deviation of the hole.